Effect of finite phosphor thickness on detective quantum efficiency

In this paper we describe theoretically the relationship between the finite thickness of a phosphor screen and its spatial‐frequency‐dependent detective quantum efficiency DQE( f ). The finite thickness of the screen causes a variation in both the total number of light quanta emitted from the screen in a burst from a given x‐ray interaction and in the spatial distribution of the quanta within the light burst [i.e., shape or point spread function (PSF) of the light burst]. The variation in magnitude of the burst gives rise to a spatial‐frequency‐independent reduction in DQE, characterized by the scintillation efficiency A S . The variation in PSF causes a roll off in DQE with increasing spatial frequency which we have characterized by the function R C ( f ). Both A S and R C ( f ) can be determined from the moments of the distribution of the spatial Fourier spectrum of light bursts emitted from the phosphor and thus they are related: A S is a scaling factor for R C ( f ). Our theory predicts that it is necessary for all light bursts which appear at the output to have the same magnitude to maximize A S and the same shape to maximize R C ( f ). These requirements can lead to the result that the fluorescent screen with the highest modulation transfer function will not necessarily have the highest DQE( f ) even at high spatial frequencies.